Recent advances toward higher output and higher efficiency in GaN related semiconductor light-emitting elements have been dramatic. In addition, active research is underway to increase efficiency of semiconductor light-emitting elements and various phosphors that use an electron beam as an excitation source. As a result, power-saving capabilities of today's light-emitting devices such as light sources, light source modules including light sources, fixtures including light source modules, and systems including fixtures are advancing rapidly as compared to their conventional counterparts.
For example, it is widely popular to incorporate a GaN related blue light-emitting element as an excitation light source of a yellow phosphor and create a so-called pseudo-white light source from a spectrum of the GaN related blue light-emitting element and a spectrum of the yellow phosphor, use the pseudo-white light source as an illumination light source or create a lighting fixture that incorporates the pseudo-white light source or, further, fabricate a lighting system in which a plurality of such fixtures are arranged in a space (refer to Patent Document 1).
Among packaged LEDs (for example, those that include the GaN related blue light-emitting element, the yellow phosphor, an encapsulant, and the like in a package material) which are a type of an illumination light source that can be incorporated into such modes, there are products with luminous efficacy of a source as a packaged LED exceeding 150 lm/W in a correlated color temperature (CCT) region of around 6000 K (refer to Non-Patent Document 2).
Furthermore, similar advances toward higher efficiency and greater power saving are being made in light sources for liquid crystal display (LCD) backlighting and the like.
However, many have pointed out that such light-emitting devices aiming for higher efficiency do not give sufficient consideration to color appearance. In particular, when used for illumination purposes, “color appearance” when illuminating an object with a light-emitting device such as a light source, fixture, system, or the like is particularly important together with increasing efficiency of the light-emitting device.
Attempts to address this issue include superimposing a spectrum of a red phosphor or a red semiconductor light-emitting element on a spectrum of a blue light-emitting element and a spectrum of a yellow phosphor in order to improve scores of a color rendering index (CRI) (CIE (13.3)) as established by the International Commission on Illumination (Commission Internationale de l'Eclairage/CIE). For example, while an average color rendering index (Ra) and a special color rendering index (R9) with respect to a vivid red color sample for a typical spectrum (CCT=around 6800 K) that does not include a red source are Ra=81 and R9=24 respectively, the scores of the color rendering indices can be improved to Ra=98 and R9=95 when a red source is included (refer to Patent Document 2).
In addition, another attempt involves adjusting a spectrum emitted from a light-emitting device particularly for special illumination applications so that color appearance of an object is based on a desired color. For example, Non-Patent Document 1 describes a red-based illumination light source.